The long-term goal of this research program is to develop safe and effective methods to prevent and treat fungal infection caused by Aspergilli called invasive aspergillosis, a leading cause of death by infection in immunocompromised patients. Three main categories of immunocompromised patients are highly susceptible to invasive aspergillosis: a) patients with severe leukopenia due to cytotoxic therapy for leukemia or lymphoma or bone marrow transplant recipients, b) organ transplant patients on high-dose corticosteroids and, c) patients with neutrophil disorders. Currently, the only available treatment is the use of antifungal agents. Even the best available agent is highly toxic, and mortality is high. An approach targeted against the basic mechanism used by the fungus to invade through the structural polymer of the host lung could yield a safer and more effective method to prevent and treat aspergillosis. It is postulated that extracellular fungal elastase is required for the fungus to invade through the elastin barrier of the lung. A combination of molecular biological and clinical animal model approaches is proposed to test this postulate and to eventually determine whether an elastase-targeted method can prevent penetration through the elastin barrier and thus prevent or limit infection. To this end, the following specific aims will be pursued: 1) test whether elastase-deficient mutant has decreased virulence in an immunocompromised murine model for aspergillosis that is restored in elastase-producing revertants, 2) isolate, purify and characterize elastase produced by Aspergillus fumigatus, 3) produce, purify and characterize antibodies against the enzyme, 4) test whether A. fumigatus penetrating into the lung of the host produces elastase, 5) clone and sequence cDNA and the gene for elastase, 6) test whether elastase gene disruption in a virulent strain of A. fumigatus results in the loss of virulence in the murine model for aspergillosis. If this approach succeeds, an analogous approach can be used for A. flavus, the other species that is known to cause aspergillosis in humans. Ultimately, the results from the proposed studies might lead to the development of a nasal spray and/or passive immunization that is effective and safe to prevent and treat aspergillosis in immunocompromised patients.